Thermoplastic polymer sheets or web material useful as surfacing materials and their manufacture

ABSTRACT

The invention relates to a process for preparing thermoplastic sheets or webs suitable for use as surfacing materials. 
     Sheets of thermoplastic polymers are known as surfacing materials and, when foamed, as substitutes for wood, but available materials do not closely resemble natural wood in appearance. The present invention provides a process whereby sheets or webs are obtained which can provide a wide variety of decorative finishes and which can approximate the appearance of such natural products as wood and marble. 
     The process of the invention comprises the extrusion of a foamable mixture of immiscible thermoplastic polymers, one of which is crystalline and of higher melting point than another, stretching the resulting foamed extrudate, compressing the stretched extrudate and then cooling the stretched and compressed extrudate. The resulting product is substantially impermeable and has integral patterning and can be used as a surfacing material in a variety of applications.

This invention relates to a process for preparing thermoplastic sheetsor webs and to the products produced thereby, the said products beinguseful as surfacing materials.

Plastics sheets comprising thermoplastic polymers are well known assurfacing materials and include, for example, polyvinyl chloride sheetsexhibiting decorative effects recognisable as imitations of natural woodand foamed thermoplastic polymers for use as wood substitutes. However,none of the known thermoplastic polymer based materials closelyapproaches, in appearance or surface texture, natural wood which has acharacteristic cellular and fibrous structure and integral, differentlycoloured, areas.

The present invention provides a process for preparing thermoplasticsheets or webs, which process comprises the steps of:

(I) EXTRUDING A FOAMABLE MIXTURE THROUGH THE DIE OF A SCREW EXTRUDER TOPRODUCE A FOAMED EXTRUDATE IN SHEET OR WEB FORM, THE FOAMABLE MIXTURECONTAINING AT LEAST A FIRST AND A SECOND THERMOPLASTIC POLYMER, THEFIRST THERMOPLASTIC POLYMER BEING SUBSTANTIALLY CRYSTALLINE AND HAVING AHIGHER MELTING POINT (AS HEREINAFTER DEFINED) THAN, AND BEINGSUBSTANTIALLY IMMISCIBLE WITH, THE SECOND THERMOPLASTIC POLYMER, AND THETEMPERATURE OF EXTRUSION BEING EQUAL TO OR GREATER THAN THE MELTINGPOINT OF THE FIRST THERMOPLASTIC POLYMER;

(II) STRETCHING THE FOAMED EXTRUDATE FROM STEP (I) IN THE DIRECTION OFEXTRUSION AS IT LEAVES THE DIE;

(III) COMPRESSING THE STRETCHED EXTRUDATE FROM STEP (II) WHILE ITREMAINS PLASTIC; AND

(IV) COOLING THE FOAMED, STRETCHED AND COMPRESSED EXTRUDATE FROM STEP(III).

We have found that by suitable selection of the ingredients of thefoamable mixture of thermoplastic polymers, by the use of suitable dyesand pigments, and by the selection of appropriate extrusion, drawing andcompression conditions we can provide thermoplastic sheets or webs whichhave a structure, appearance and texture unlike those known hitherto andwhich can approximate in appearance and texture to wood veneers or othernatural products having pattern striations. Thus, by the process of theinvention we provide substantially impermeable thermoplastic sheets orwebs with integral patterning. This integral patterning is created bydifferences in texture or reflectivity between adjacent areas of thesheet or web arising from the presence therein of immisciblethermoplastic polymers and can be enhanced by colouring the adjacentareas differently. The first and second polymers (which may bedifferently coloured) do not form an homogeneous mixture during theprocess and hence may be distinguished from each other in the resultingproduct. Because of their different physical properties and optionallydifferent colours, the different polymers together confer upon theproduct a decorative effect, which effect may be utilised by selectionof appropriate ingredients and processing conditions to provide productswhich closely simulate natural wood, or marble or other naturalmaterials.

Where herein we use the term "melting point" in relation to acrystalline polymer, we mean that temperature at which the polymerchanges state from a solid to a liquid and where we use the term inrelation to a non-crystalline polymer we mean that temperature commonlyreferred to as the glass transition temperature.

Whilst it is within the scope of our invention to employ substantiallycrystalline polymers as both the first and second thermoplasticpolymers, we prefer to employ foamable mixtures comprising anon-crystalline polymer as the second thermoplastic polymer.

For the sake of simplicity, our invention will hereinafter be describedby reference to sheets or webs comprising two only thermoplasticpolymers, namely the first substantially crystalline thermoplasticpolymer of higher melting point than the second, preferablynon-crystalline, thermoplastic polymer. It should be understood,however, that similar considerations to those discussed apply also tosheets and webs comprising three or more thermoplastic polymers wherethe context so allows, and that the preparation of such products iswithin the scope of our invention.

In performing the process of our invention, we prefer to form a foamablemixture of the first and second thermoplastic polymers and any otheringredients such as fire-retardants, stabilisers, fillers, anti-staticagents and colourants prior to the introduction of the mixture into theextruder. The ingredients of the foamable mixture thus used may be inany suitable physical form such as beads or granules. We prefer thethermoplatic constituents to be in granular form, especially where theproduct is desired to exhibit a striated appearance.

Any or all of the polymers in a foamable mixture may be pre-compoundedwith a suitable blowing agent or, in a less preferred embodiment, atleast one suitable blowing agent may be mixed with first and secondpolymers before or after they are placed in the extruder. Gas underpressure may be injected into a mixture of the first and secondthermoplastic polymers in the extruder to act as, or to supplement, ablowing agent. Further, those ancilliary materials known for use in theproduction of uniform extruded foams as "cell size control additives"may be incorporated into the mixture prior to extrusion.

We prefer, however, to utilise mixtures wherein at least one of thefirst and second thermoplastic polymers incorporates a blowing agentand, more especially, to employ as the second thermoplastic polymer anon-crystalline polymer incorporating a blowing agent together with asubstantially crystalline first thermoplastic polymer which does notincorporate such an agent; optionally also, further non-crystallinepolymers not incorporating a blowing agent may be included.

Any suitable blowing agent may be used provided it produces a foamedextrudate at the temperature of the extrusion step (i). The blowingagent may thus be a compound which decomposes to provide gaseous orvolatile decomposition products at the temperature of extrusion, or maybe a compound which vaporises at the temperature of extrusion, or may beas mentioned above, a gas introduced under pressure into a mixture offirst and second thermoplastic polymers in the extruder. Blowing agentsfor use in preparing foamed thermoplastic polymers are well known andany suitable known agent can be used. The blowing agent should bepresent in sufficient amount as to ensure satisfactory foaming. We havefound that foamable mixtures incorporating up to about 5% by weight ofblowing agent may be satisfactorily employed, but we prefer to useamounts up to about 1% by weight and still more preferably up to about0.5% by weight of the blowing agent, the amount of blowing agent beingbased on the weight of the foamable mixture.

Suitable first and second thermoplastic polymers are known.Non-crystalline polymers which can be used as the second thermoplasticpolymer include cellulose acetate, cellulose propionate, celluloseacetate butyrate, ethyl cellulose, polystyrene, styrene-acrylonitrilecopolymers, polycarbonates, styrene and methyl styrene copolymers andphenylene oxide polymers. Substantially crystalline polymers which canbe used as the first, and optionally also as the second, thermoplasticpolymer include high density polyethylene, polypropylene, polybutene-1,poly 4-methyl pentene-1, polyethylene terephthalate and nylon 6(polycaprolactam), nylon 66 (poly(hexamethyleneadipamide)) and nylon 11(polyundecanoamide).

We prefer to employ polystyrene and polypropylene mixtures; morepreferably we employ a major proportion of polystyrene and a minorproportion of polypropylene. Still more preferably we employ 60 to 95parts of polystyrene and 40 to 5 parts of polypropylene per 100 parts byweight of the thermoplastic components of the mixture. In the mostpreferred embodiment, we employ both the polystyrene and polypropylenein granular form with only the polystyrene granules incorporating ablowing agent.

As is well known, thermoplastic polymer materials are availablecommercially in coloured and uncoloured varieties and we select thecolour or colours of our material in accordance with the visual effectwe desire in our product. Thus if we wish to provide a thermoplasticpolymer sheet or web exhibiting a single colour, the colourants employedmay be mixed with the polymers prior to extrusion. If we wish to providea product with a variegated colour appearance, then we employ acolourant or colourants pre-incorporated into one or more of thepolymeric components. More especially, if a striated colour effectsimilar to that observed in natural wood is desired, we prefer to employgranules of the second thermoplastic polymer comprising a colourantcorresponding to the desired background colour with some, at least, ofthe granules of the first thermoplastic polymer comprising a relativelyhigh proportion of the same or a different colourant, optionally withgranules of at least one other high melting point polymer containing arelatively lower proportion of the same or a different colourant.

When employed, the size and number of the polymer granules comprising ahigh proportion of colourant determines, at least in part, thedecorative effect of the products sheets or webs. By the operation ofour process, streaks and lines, corresponding generally with the numberand size of the highly pigmented particles, are created in the product;smaller granules give generally narrower streaks than the largergranules and hence products having a desired striated appearance can beproduced by suitable selections of the size and number of highlycoloured granules incorporated in the foamable mixture.

The following description of the performance of our invention will bemade with reference to the accompanying FIGURE.

The FIGURE is a schematic representation of an extrusion apparatussuitable for carrying out the process of our invention.

The FIGURE shows an extruder 1 which may be, for example, a conventionalsingle screw extruder having an L:D (length to diameter) of about 20:1and a compression ratio of about 2:1. The extruder is fitted with acontrollable heating means and a suitably dimensioned die 2. The die 2is preferably a conventional "coat hanger" sheet die fitted with arestrictor bar and adjustable die lips (by means of which a generallycylindrical feed from the extruder barrel is caused to flatten and forma substantially planar web or ribbon of extrudate). The die size isdetermined in accordance with the desired dimensions of the product.

By suitable operation of the extruder 1 with regard to feed rate, screwspeed and temperatures, the foamable mixture therein is heated andforced out through the die 2 as extrudate 6. The mixture in the extruderis heated so that its temperature in the barrel and die is equal to orgreater than (but not significantly in excess of) the meltingtemperature of the first thermoplastic polymer. The temperature of theextrudate 6 leaving the die 2 is preferably such that, in conjunctionwith the stretching action to which it is then subjected, most of thecells of the foam burst allowing escape of the gas or vapour produced bythe blowing agent.

Because of the extensive collapse of the foam structure, the density ofthe product is considerably greater than that of conventional foamedextrudates and approaches that of unfoamed thermoplastic polymers.

The stretching and compression of the extruded foamed or foaming webemerging from the die is performed by suitable nip means which, byfriction contact with the major surfaces of the web, draw it away fromthe die slit. We prefer the suitable nip means to comprise a pair ofdriven metal rollers 3 mounted on horizontal axes and verticallydisposed in relation to each other so as to define a gap parallel to thedie slit and so driven in contra-rotation at a selected speed that thefrictional force exerted by them on the extrudate causes the desiredstretching. The preferred rollers are provided with suitable coolingmeans whereby they may be maintained at a desired temperature and aremounted so as to be adjustable to provide between them the said gapwhich, being less than the thickness of the foamed extrudate approachingthe rollers, acts as a nip with the result that the extrudate iscompressed as it passes through the gap.

By suitable operation and control of the extruder, the die lips andrestrictor bar settings, the temperature of the extrudate, the niproller drive speed and the nip roller spacings, the surface texture andappearance of the web leaving the stretching/compression means may bevaried. As has been stated above, the effect of the stretching on thefoamed extrudate in combination with its temperature is such that mostof the foam cells are ruptured, allowing escape to the atmosphere of gasor vapour from the blowing agent; further, the walls of the collapsedcells become elongated in the direction of stretch and an orientation isimparted to the thermoplastic materials. As a consequence of thestretching, the web is caused to take up a fibrous structure which maybe more or less marked as determined by the degree of stretch employedand, in a preferred embodiment, the process is performed and controlledso that the product web has a texture and appearance of natural wood.

The texture and appearance of the product web may also be varied in acontrolled reproducible manner by forming and maintaining, byalterations in suitable variables in the machine and process, a surplusof the extrudate upon the web at the nip formed by the rollers. Thissurplus material forms a "rolling bank" at the inlet to the gap betweenthe rollers and modifies the surface of the web with which it is incontact, the extent of the modification being determined in part by thesize and shape of the rolling bank.

The extrudate leaving the die slit is at a temperature substantially thesame as that maintained in the die; however, after emerging from theslit the temperature rapidly falls due to heat losses to the atmosphereand loss of blowing agent. The distance between the die slit and the nipmeans should be such that when the extrudate reaches the latter it isstill in a plastic state and thus its surface can be modified andconsolidation effected. We prefer that the spacing between the nip meansand the die slit is variable so that the plastic consistency of theextrudate entering the nip is that which has been determined by previoustrial to be the optimum. By adjustment of this spacing and thetemperature of the rollers comprising the nip means, close control ofthe plasticity of the extrudate in the nip area can be maintained.Control of this plasticity is important in that it determines, at leastin part, the texture and appearance of the product sheet or web; if theextrudate were to be too plastic, then the desired fibrous structurewould be substantially lost whilst if the extrudate were to beinsufficiently plastic, then the extrudate would recover substantiallyfrom the compression imparted by the nip rollers.

The web, after stretching and compression is further cooled, preferablyunder tension, suitably by passage through a cooling zone 4 which may be(but cooling means such as chilled rollers may optionally be used) andthen either reeled, optionally, for later cutting into sheets of thedesired dimensions, or cut into desired sheets without reeling. Duringthis further cooling step, the web may be maintained under tension forexample by suitable nip means such as a pair of haul-off rollers 5, asshown in the FIGURE.

The thermoplastic sheets or webs produced according to the process ofthe invention may be affixed to substrates using suitable knownadhesives customarily employed for adhering thermoplastic materials tothe desired substrates. For example, the sheets or webs may be adheredto chipboard substrates by urea-formaldehyde or polyvinyl acetateadhesives using conventional presses and operating temperatures.

The thermoplastic sheets or webs can be sanded (suitably after adhesionto the desired substrate) using conventional sanding techniques andapparatus. Further, the sheets or webs whether sanded or not can bestained, for example with conventional wood stains, and can be varnishedwith natural or synthetic resin varnishes to provide a desired surfacefinish.

The width of the sheets and webs that may be prepared according to theprocess of the invention is limited only by the dimensions of the die,nip means and ancilliary equipment available. The thickness of thesheets and webs may be varied over a wide range but we prefer to prepareproducts from 0.2 mm to 2.0 mm thick and, more preferably, 0.4 to 1.0 mmthick. The sheets and webs in the more preferred range are convenient tohandle and apply and are economic to produce.

In order that the invention may be more clearly understood, it isfurther illustrated by the following Examples.

EXAMPLE 1

A thermoplastic sheet having an appearance and texture approximating toa natural wood was prepared as follows:

400 parts by weight of polystyrene granules (Sterling ST. 153) (Natural)

150 parts by weight of polypropylene granules (I.C.I. "Propathene"GWE.21) (Natural).

200 parts by weight of expandable polystyrene ("Styropor" P.455)(Natural). and

1 part by weight of polypropylene granules (Black Masterbatch I.C.I"Propathene" C4/905 Ebony Black).

("Styropor" and "Propathene" are Registered Trade Marks).

were mixed and charged to a 60 mm single screw extruder having a 22:1L/D ratio and a 2:1 compression ratio screw operating at a screw speedof 40 r.p.m. The outlet part of the extruder was coupled directly to aslit die of "coathanger" construction fitted with a restrictor bar andadjustable die nips. The mouth of the slit was 500 mm wide and was setto a slit height of 2 mm.

The heaters on the extruder were set to produce temperatures in the fourzones of the barrel of the extruder and in the die of 140°, 150°, 160°,170° and 180° C. respectively.

On operation of the extruder, the mixture was forced through the slitand foaming of the extrudate occurred. The foamed extrudate was drawnaway from the slit by nip means positioned 400 mm from the die slit andcomprising a pair of 660 × 120 mm hard chrome plated metal rollersmounted on horizontal axes and provided with a cooling water circulationsystem. The rolls were hydraulically loaded to give a roll pressure ofabout 1360 kg. and were driven in contra-rotation to each other at thesame surface speed of 10 ft/min (3.05 m/min).

The rolls were so positioned that their adjacent surfaces were 0.5 mmapart and were maintained at a temperature of 15° C. The speed ofrotation of the rollers was such that by means of frictional contact thefoamed extrudate was stretched, in the direction of extrusion, toproduce an elongation of about 3 times. The stretching, together withthe temperature and plasticity of the foam, was such that substantiallyall the foam bubbles burst and the thermoplastic components wereoriented parallel to the stretch direction to give a fibrous structureprior to the extrudate entering the nip between the rollers.

The nip was so constructed and operated that a surplus of the extrudatein the form of a rolling bank (a cylinder of material extending thewidth of the web and of approximately 15 mm diameter) was built up andmaintained upon the upper surface of the extrudate adjacent to the upperroller.

The stretched and correspondingly narrowed extrudate was compressed bypassage through the nip to form a web having a width of the same orderas the width of the die slit (500 mm), and then cooled by traversing aspace of 800 mm to a haul-off unit comprising a pair of "Neoprene"("Neoprene" is a Registered Trade Mark) coated rolls having the samedimensions as the stretch rolls and driven at the same speed so that theextrudate entering the nip between the haul-off rolls was held underslight tension. The material, which was about 1 mm thick, emerging fromthe nip between the haul-off rolls was then reeled.

Sheets of the desired size were cut from the reel and exhibited a silverlustre with integral black striations which together with its fibrillarstructure closely approximated in texture to a wood veneer.

EXAMPLE 2

400 parts by weight polystyrene granules (Sterling ST. 153 Natural).

150 parts by weight polypropylene granules (I.C.I. "Propathene" GWE.21Natural).

200 parts by weight expandable Polystyrene + brown pigmentation(Sterling EX. 4791. Y)

0.8 parts by weight Black Masterbatch ("Propathene" C4/905 Ebony Black)

2.0 parts by weight Red Masterbatch ("Propathene" C4/401 Guardsman Red)

20 parts by weight Yellow Masterbatch ("Propathene" C4/201 CrocusYellow)

were mixed by tumble blending and processed as described in Example 1,except that the die was maintained at a temperature of 170° C., and theseparation of the nip rollers was such as to maintain a rolling bankabout 2 mm in diameter.

The web obtained (which was about 0.6 mm thick) had a matt surfacepatterned with fissures and coloured striations which together gave theweb the appearance and feel of an open grained wood veneer. Sheets werecut from the web and adhered to a chipboard substrate by means of aconventional polyvinyl acetate adhesive.

EXAMPLE 3

400 parts by weight Polystyrene granules (Sterling ST.153 Natural)

150 parts by weight Polypropylene granules (I.C.I. "Propathene" GWE. 21Natural)

200 parts by weight Expandable polystyrene + light brown pigmentation(Sterling EX. 4146.Y)

0.4 parts by weight Black Masterbatch ("Propathene" C4/905 Ebony Black)

0.5 parts by weight Red Masterbatch ("Propathene" C4/401 Guardsman Red)

40 parts by weight Yellow Masterbatch ("Propathene" C4/201 CrocusYellow)

were mixed by tumble blending and processed as described in Example 1,except that the die temperature was maintained at 190° C. and theseparation of the nip rollers was such as to maintain a rolling bankabout 2 mm in diameter.

The web obtained (which was about 0.6 mm thick) had a smooth surfacepatterned with coloured striations which gave a good approximation to apolished wood veneer.

EXAMPLE 4

400 parts by weight Polystyrene granules (Sterling ST.153 Natural),

150 parts by weight Polypropylene granules (I.C.I. "Propathene" GWE.21)

200 parts by weight Expandable Polystyrene + dark brown pigmentation(Sterling EX. 4793)

2 parts by weight Black Masterbatch ("Propathene" C4/905 Ebony Black).

10 parts by weight Red Masterbatch ("Propathene" C4/401 Guardsman Red)

were mixed by tumble blending and processed as described in Example 1.

The web obtained had a rough surface texture and woodlike charactercombined with an attractive abstract patterning. The web was cut intosheets of desired size and then bonded to planks of chipboard using aconventional urea-based adhesive. After smoothing the sheet surface bypassage through a mechanical sander, a finishing treatment with "buttonpolish" was applied to provide the composite panel with a smoothpolished wood-plank appearance.

EXAMPLE 5

60 parts by weight polycarbonate granules (Engineering Polymers Ltd,Lexan 2014 Natural)

10 parts by weight polypropylene granules (I.C.I. "Propathene" GWE 21Natural)

0.4 parts by weight blowing agent (Fisons Ltd., Genitron EPB)

were mixed by tumble blending.

The mixture was processed as described in Example 1, except that barreland die heating zones were set at 150°, 160°, 170°, 180° and 190° C.respectively, the die opening adjusted to 3.0 mm and the rolling bank ofmaterial in the die nip maintained at about 5 mm diameter by adjustingthe speed of the nip rolls. The web obtained, which was about 1.4 mmthick, exhibited a variegated reflective appearance.

EXAMPLE 6

60 parts by weight Polystyrene granules (Sterling ST. 153 Natural).

10 parts by weight Nylon 11 granules (Aquitaine Total Organica S.A..,Rilsan BENO)

0.4 parts by weight blowing agent (Fisons Ltd., Genitron EPB)

were mixed by tumble blending.

The mixture was processed as described in Example 1, except that thebarrel and die heating zones were set at 160°, 170°, 180°, 195° and 200°C. respectively, the die opening adjusted to 3.0 mm and the rolling bankof material maintained at about 5 mm diameter by adjusting the speed ofthe nip rolls. The sheet obtained was yellowish/white in colour andexhibited a variable pattern of light and dark areas.

EXAMPLE 7

50 parts by weight Polystyrene granules (Sterling ST.153 Natural)

10 parts by weight polystyrene granules containing blowing agent(Sterling ST.153/SF Natural)

10 parts High Density polyethylene granules (Wacker DF 5074G)

were mixed by tumble blending.

The mixture was processed as described in Example 5 to give a web about1.1 mm thick which was white in colour and exhibited a pattern of lightand dark shading.

The web was cut into sheets and employed as wall-cladding material.

EXAMPLE 8

50 parts by weight Polystyrene granules (Sterling ST. 153 Natural)

10 parts by weight polystyrene granules containing blowing agent(Sterling ST.153/SF Natural)

10 parts by weight polypropylene (I.C.I. "Propathene" GWE 21 Natural)

0.01 parts by weight yellow pigment (PV Fast Yellow)

0.01 parts by weight blue pigment (BV Fast Blue)

were mixed by tumble blending.

The mixture was processed as described in Example 5 to give a web about1.1 mm thick was emerald green in colour and featured a pattern offibrous reflective element against a darker background. The web was cutinto sheets which were adhered to a chipboard substrate useful as apartitioning material.

EXAMPLE 9

50 parts by weight polystyrene granules (Sterling ST. 153 Natural)

10 parts by weight polystyrene granules containing blowing agent(Sterling ST. 153/SF Natural)

10 parts by weight polypropylene granules (I.C.I. "Propathene" GWE. 21Natural).

0.15 parts by weight Black Masterbatch ("Propathene" C4/905 Ebony Black)

15 parts by weight fire retardant (DOW Chemicals Ltd, FR300 BA)

5 parts by weight Antimony Oxide

0.45 parts by weight light stabiliser (Ciba-Geigy Ltd. Tinuvin 327)

0.225 parts by weight antioxidant (Ciba-Geigy Ltd Irganox 1076)

were mixing by tumble blending. The mixture was processed as describedin Example 5, except that the die was maintained at a temperature of200° C.

The web obtained, which was about 1 mm thick, was patterned with afibrous striation giving it the general appearance of wood. Whensubjected to the oxygen index test according to ASTM D 2863/70 a valveof 30.5 was obtained. This compared with the value of less than 26obtained with a product prepared in the same manner but without theinclusion of FR 300 BA and Antimony Oxide. The web was cut into sheetseach of which was adhered to a chipboard substrate and used to fabricatefurniture.

EXAMPLES 10, 11 and 12

To exemplify the use of mixtures of crystalline polymers, the followingblends were mixed by tumbling and then extruded as described in Example9.

    ______________________________________                                        EXAMPLE NO.    10        11        12                                         ______________________________________                                        Polypropylene granules                                                                       40 parts  10 parts  40 parts                                   (I.C.I. "Propathene"                                                                         by        by        by                                         GWE. 21)       weight    weight    weight                                     High density Poly-                                                                           10 parts                                                       ethylene granules                                                                            by        --        --                                         (Wacker-Chemie weight                                                         G.m.b.H DF 5074G)                                                             Nylon 11 granules        40 parts  10 parts                                   (A.T.O. Rilsan --        by        by                                         BENO)                    weight    weight                                     Genitron E.P.B.                                                                              0.5 parts 0.5 parts 0.5 parts                                                 by        by        by                                                        weight    weight    weight                                     ______________________________________                                    

The webs obtained were translucent and in the thickness range 0.6-0.9mm. Each of the products from Examples 10, 11 and 12 exhibited adistinctive patterning comprising variegated striations, the nature ofthe pattern obtained being different for each of the products.

We claim:
 1. A process for preparing thermoplastic sheets or webs, whichprocess comprises the steps of:(i) extruding a foamable thermoplasticpolymer mixture through the die of a screw extruder to produce a foamedextrudate in sheet or web form, the foamable thermoplastic polymermixture containing at least a first and a second thermoplastic polymer,the first thermoplastic polymer being substantially crystalline andhaving a higher melting point than, and being substantially immisciblewith, the second thermoplastic polymer, and the temperature of extrusionbeing equal to or greater than the melting point of the firstthermoplastic polymer; (ii) stretching the foamed extrudate from step(i) in the direction of extrusion as it leaves the die to rupture mostof the cells of the foamed extrudate and to elongate the walls of thecollapsed cells in the direction of stretch; (iii) compressing thestretched extrudate from step (ii) while it remains plastic; and (iv)cooling and foamed, stretched and compressed extrudate from step (iii).2. A process according to claim 1 wherein the die of the extruder is asheet die fitted with a restrictor bar and adjustable die lips and theextrudate is stretched in step (ii) and compressed in step (iii) bypassing it, whilst it remains plastic, through co-operating nip meanscomprising a pair of driven metal rollers mounted on horizontal axes andvertically disposed in relation to each other so as to define a gapparallel to the die slit and driven in contra-rotation.
 3. A processaccording to claim 2 wherein the gap between the rollers and their speedof rotation is such as to maintain a surplus of the extrudate upon thesheet or web adjacent the nip.
 4. A process according to claim 1 whereinsheets or webs are produced having a thickness of 0.4 mm to 1.0 mm.
 5. Aprocess according to claim 1 wherein at least one of the first andsecond thermoplastic polymers is introduced into the extruder in theform of beads or granules and at least some of the beads or granulescontain a blowing agent.
 6. A process according to claim 5 wherein thefoamable thermoplastic polymer mixture has been formed by mixing beadsor granules of the second thermoplastic polymer containing a colourantcorresponding to the background colour desired in the product sheet orweb with beads or granules of the first thermoplastic polymer containinga higher proportion of the same or a different colourant.
 7. A processaccording to claim 1 wherein the second thermoplastic polymer is anon-crystalline thermoplastic polymer.
 8. A process according to claim 7wherein the first thermoplastic polymer is selected from high densitypolyethylene, polypropylene, polybutene-1, poly 4-methylpentene-1,polyethylene terephthalate, nylon 6, nylon 66 and nylon 11 and thesecond thermoplastic polymer is selected from cellulose acetate,cellulose propionate, cellulose acetate butyrate, ethyl cellulose,polystyrene, styrene-acrylonitrile copolymers, polycarbonates, styreneand methyl styrene copolymers and phenylene oxide polymers.
 9. A processaccording to claim 8 wherein the foamable thermoplastic polymer mixturecomprises 60 to 95 parts of polystyrene and 40 to 5 parts ofpolypropylene per 100 parts by weight of the thermoplastic polymercomponents of the foamable mixture.
 10. A process according to claim 1wherein the foamable thermoplastic polymer mixture contains up to 1% byweight of blowing agent.
 11. A process according to claim 1 whichincludes the additional step of sanding one or both surfaces of theextrudate subsequent to the cooling step (iv).
 12. A process accordingto claim 1 which includes the additional step of varnishing or stainingthe extrudate subsequent to the cooling step (iv).
 13. The thermoplasticsheet or web having an integral patterning and fibrous structureprepared in accordance with the process of claim
 1. 14. A surfacingstructure comprising the sheet or web product of claim 13 bonded to asubstrate by an adhesive.